The invention pertains to the field of communications interfaces via which DC power is provided to operating circuitry.
Powered communications interfaces are utilized in data communications systems to provide operating power to devices over the same wires used to carry data communications signals, in a manner analogous to the traditional telephone system in which DC operating power is provided to subscriber equipment over the twisted-pair telephone wires. Today, there is widespread use of so-called “power over Ethernet” or POE technology, in which DC operating power is provided to digital telephones and other data equipment over unshielded twisted pair (UTP) cables connecting the data equipment with centralized data switches. In POE parlance, a device receiving power in this fashion is termed a “powered device” or PD, while a device that provides power for use by PDs is termed a “power sourcing equipment” or PSE.
An Ethernet communications interface utilizes transformer coupling of transmitted and received data signals in order to maintain electrical isolation between devices that are connected together by an Ethernet cable. DC circuitry (such as power source or load) is connected between respective center-taps of interface-side windings of transmit and receive transformers. Current flowing from the DC circuitry to an interface-side winding of one transformer is split into two components that flow through respective conductors of a twisted pair of the Ethernet cable, and likewise current flowing into the DC circuitry from the interface-side winding of the other transformer is the result of the adding together within the interface-side winding of two current components received from respective conductors of another twisted pair. Ideally, the respective paths of the current components are substantially matched so that the current splits into two substantially equal portions.
It is possible, however, that in any given system there is a mismatch or imbalance between the two current components of a powered communications interface, and if the imbalance is severe enough then problems may occur in the operation of the data communications interface. Such imbalance generally occurs due to a mismatch between the overall electrical resistances of the respective paths traveled by the current components. Such differences in resistance can occur in transformers, cables, connectors, and patch panels for example. One effect of this type of imbalance is the possibility of distortion on the data signals passing through the transformers, which if severe enough can cause data errors or even render the communications link unusable. The distortion, which is referred to as “droop”, arises because of the magnetizing effect of the mismatched currents flowing through the transformer. The interface-side winding of the transformers is wound such that when equal DC current flows through them, the net magnetizing effect is zero. When the currents are unequal, the operating point of the transformer is shifted away from a desired zero-magnetization operating point, and pulse droop may result.
US Published Patent Applications 2006/0115007 A1 and 2006/0119478 A1 show a circuit delivering common mode inline power over a pair of conductors, in which any imbalance in the current carried by the conductors is detected and compensated with a bias current applied to counter the imbalance. The droop may be measured by coupling a receiver to the transmitter output at the physical-layer circuitry (PHY). The transmitter transmits a differential AC signal into the primary of the transformer. A receiver is coupled to receive and monitor the signal transmitted by the transmitter. A processor (or other suitable circuit) determines if there is droop by comparing the (possibly) distorted pulse (or characteristics thereof) to an expected or ideal pulse (or characteristics thereof). This may be done, for example, by measuring the peak amplitude or the amplitude at some point in the pulse, for example, near the end of the pulse where the droop usually tends to be more pronounced. This amplitude is compared to the expected or desired amplitude and a difference error signal related to the magnitude of the droop is generated. This is applied to a feedback loop which applies a correcting DC current to an appropriate node of the circuit in order to counter the unbalanced current flow on the wire side of the transformer and thereby bring the difference error signal to zero or nearly zero and so reduce the droop and correct the shape of the pulse.